1. Field of the Invention
The present invention relates to techniques for establishing fluid communication between a subterranean formation and a downhole tool positioned in a wellbore penetrating the subterranean formation. More particularly, the present invention relates to probes and associated techniques for drawing fluid from the formation into the downhole tool.
2. Background of the Related Art
Wellbores are drilled to locate and produce hydrocarbons. A downhole drilling tool with a bit at an end thereof is advanced into the ground to form the wellbore. As the drilling tool is advanced, a drilling mud is pumped through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The fluid exits the drill bit and flows back up to the surface for recirculation through the tool. The drilling mud is also used to form a mudcake to line the wellbore.
During the drilling operation, it is desirable to perform various evaluations of the formations penetrated by the wellbore. In some cases, the drilling tool may be provided with devices to test and/or sample the surrounding formation. In some cases, the drilling tool may be removed and a wireline tool may be deployed into the wellbore to test and/or sample the formation. These samples or tests may be used, for example, to locate valuable hydrocarbons.
Formation evaluation often requires that fluid from the formation be drawn into the downhole tool for testing and/or sampling. Various devices, such as probes, are extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and draw fluid into the downhole tool. A typical probe is a circular element extended from the downhole tool and positioned against the sidewall of the wellbore. A packer at the end of the probe is used to create a seal with the wall of the formation. The mudcake lining the wellbore is often useful in assisting the packer in making the seal. Once the seal is made, fluid from the formation is drawn into the downhole tool through an inlet in the probe by lowering the pressure in the downhole tool. Examples of such probes used in wireline and/or drilling tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045 and 6,609,568 and U.S. patent application Ser. No. 2004/0000433.
Despite the advances in probe technology, there remains a need for a reliable probe that is capable of operating in extremely harsh wellbore conditions. During operation, the seal between the packer and the wellbore wall may be incomplete or lost. When a probe fails to make a sufficient seal with the wellbore wall, problems may occur, such as contamination by wellbore fluids seeping into the downhole tool through the inlet, lost pressure and other problems. Such problems may cause costly delays in the wellbore operations by requiring additional time for more testing and/or sampling. Additionally, such problems may yield false results that are erroneous and/or unusable.
There also remains a need for a probe that routinely provides an adequate seal with the formation, particularly in cases where the surface of the well is rough and the probe may not have good contact with the wellbore wall. It is desirable that such a probe be provided with mechanisms that provide additional support to the packer to assure a good seal with the wellbore wall. Moreover, it is desirable that such a probe conforms to the shape of the wellbore, distributes forces about the probe and/or reduces the likelihood of failures.